Wireless technologies have brought about conveniences and benefits to many people's daily lives as devices such as cell phones, notebooks, and tablet PCs, have enabled users to communicate with each other from just about any location and to make resources on the internet more easily accessible. Among various wireless technologies, Device-to-Device (D2D) communication has drawn more and more attention since D2D communication, unlike the traditional cellular communication system, does not require information exchanges to always pass through a base station and a core network upon transmission as users communicating in a D2D mode could make direct link establishment and transmission between devices.
While a conventional cellular communication requires resources allocated for both uplink (UL) and downlink (DL), D2D communication could only require resources allocated for a direct link. Also for a convention cellular system, resources are dedicated to a device within the coverage of a cell for a particular point in time; whereas for D2D communication, resources could be allocated to different pairs of devices at a particular point in time as long as each pair of devices would cause no interferences between other pairs of devices engaging in D2D communication.
Since a conventional cellular communication system requires a base station to deliver UL and DL data from one device to another while D2D communication has one direct link, D2D communication would result in less data transmission overall. In a conventional cellular communication system, there would be uplink and downlink radio links transmission delay as well as the transmission delay in the core network and even other networks. However, in D2D communication, only one radio link transmission delay would exist normally.
Because of the improved resource utilization and the reduced transmission delay in D2D communication as previous mentioned, both network providers and consumers would benefit from D2D communication. Efficient resource utilization in D2D communication could translate to more users served by the network providers, and the benefit of having less round trip delay could include uninterrupted conversations and more pleasant feelings during a conversation.
Methods of D2D connection establishment could be roughly divided into two categories. One category would be to establish D2D connections by devices themselves, and the other category is to establish D2D connections with the assistance of a network. For the former category, one device may send a signal to detect neighboring devices, and then a nearby device may receive the detection signal. After device synchronizations and coordination are finished, devices may communicate with each other directly, and thus D2D communication is established by devices themselves. As for the latter category, devices would synchronize with each other with the help of the network, or otherwise the devices could also synchronize with each other by using default parameters or by negotiating parameters agreeable to both devices through the network. Once the network assisted synchronization and coordination is finished, a direct communication between devices could be performed under the guidance of the network.
Unlike a conventional cellular communication system in which devices could communicate regardless of the distances between them, D2D communication would less likely allow communications over long distances. For this reason, the aforementioned D2D connection establishment method may require a device to possess a foreknowledge of the presence of other devices nearby as well as explicit indications as to whether these devices have activated their D2D communication capabilities. Otherwise, a blind initiation to attempt D2D communications would not only consume the battery power of the device itself but also waste radio resources of the network. Therefore, when one wants to communicate with another user without knowing the proximity of such user, the person usually would make a normal call through the conventional cellular communication means. However, in many situations, people could practically be right by each other but at the same time not being aware of such fact and thus may dial another user in proximity through the conventional cellular communication means without taking D2D communication.
A method has to be proposed for a device to automatically perform D2D communication if such capability exists and is activated in the device without making mandatory the foreknowledge of the proximities of other devices or the explicit indications for whether other devices and would be capable and willing to participate in D2D communications.